As a certain kind of light source unit used for supplying lighting light to, for example, an endoscope, has heretofore been mentioned a unit equipped with a light source 20 having a discharge lamp 21 and a converging mirror 25 converging light from the discharge lamp 21, and a light guide 52, on which the light from the light source 20 is incident, as illustrated in FIG. 5, and having a construction that these light source 20 and light guide 52 are held and fixed at respective predetermined positions in a casing 11, the light from the discharge lamp 21 reflected on the converging mirror 25 is struck on the light guide 52 by condensing the light at a condensing point located at an arrangement position of a light incident port 53 of the light guide 52, and the light is guided by this light guide 52 and emitted outside (see, for example, Patent Literature 1).
In the embodiment illustrated in this drawing, reference sign 40 is a light shielding body supported by a support 48 for light shielding body fixed to the casing 11 and surrounding an optical path L1 from the light source 20 to the light guide 52 of the light emitted from the discharge lamp 21 and converged by the converging mirror 25 and is also a stray light preventing member for preventing light toward other directions than the direction of the light incident port 53 of the light guide 52, which is generated in the interior of the casing 11, for example, light which travels on an optical path indicated by sign L2, from going out of the casing 11 as stray light. Reference sign 17 is a light quantity adjusting member for adjusting the quantity of the light incident on the light guide 52 and having a construction that a plurality of light passage apertures different in aperture diameter is formed in a fan-shaped light shielding plate 17A, and a light passage aperture 17B having an aperture diameter of a proper size is selected and located on the optical path.